Rapid advances in the technology of printed circuits and consequent miniaturization have created a growing demand for appropriate mass soldering methods. More components are being concentrated within a printed circuit board than ever before, and due to the increased number of soldered joints and their corresponding close spacing, reliable mass soldering has become increasingly critical.
The conductive method, whereby component assemblies are heated on a "hot plate," is a low-volume, high energy-consuming, difficult to control method of accomplishing solder reflow. The conductive method is ineffective in processing the latest in hybrid assemblies as well as being potentially damaging to temperature-sensitive assemblies.
The convective method involves directing high-velocity heated air at the assembly. This process is slow and energy inefficient, imprecise, and also potentially damaging to heat-sensitive components because, like conductive methods, all the components reach the maximum temperature required to accomplish solder reflow.
Vapor phase, a relatively new method of accomplishing solder reflow, utilizes direct contact condensation heating. The assembly to be heated is immersed in an atmosphere of vapor generated by a pool of boiling fluorocarbonated liquid. The vapor, at the boiling point of the liquid, envelopes the assembly and begins to condense, giving up its latent heat of vaporization and raising the temperature of the assembly to that of the boiling point of the liquid. This causes the solder to melt and reflow.
An advantage of the vapor phase method is temperature control specific to the boiling point of the liquid, such that over-heating is impossible. However, temperatures above 253.degree. C. are unobtainable because fluorocarbonated liquids have not yet been developed capable of boiling at higher temperatures. Furthermore, production is limited to the specific temperature of the liquid, i.e., if one wants to process a product with a solder having a different reflow temperature, the liquid must be drained and new liquid with the required boiling point charged into the system. This results in down time and excess fluid cost. Successful processing of assemblies with more than one solder type or temperature requirement is virtually impossible without processing the assembly more than once through different liquids.
The simple fact that vapor phase heats by pure conduction, i.e., direct contact condensation, is a disadvantage once again because the entire assembly must be heated to the reflow temperature of the solder. Predrying of the solder paste is also necessary or solder spattering can result, and shifting/misalignment of components.
Early infrared conveyer ovens utilized focused tungsten filament lamps to accomplish surface bonding onto ceramic substrates. These types of ovens however were not successful in surface mounting to epoxy/glass or polyimide/glass printed circuit boards.
Focused emitters typically emit short wave infrared radiation in the near or middle infrared regions. The effect of this shortwave emission is twofold. First, the reflective and color-selective nature of these short wavelengths, along with the varied material geometries and thermal conductivities of the components and substrates, causes large .gradient. T's between component (differences in temperature) and substrate at reflow point. Large .gradient. T's can cause flux charring, charring of polymeric assemblies, and damage to temperature-sensitive components.
Secondly, energy consumption is high. Focused bulb-type emitters operate at temperatures up to 2,700.degree. K. and use up to 1,000 watts each. Some systems use up to 60 bulbs.
A typical furnace utilizing focused infrared lamp emitters consists of a tunnel having a process area approximately 30 inches long and constructed with alumina/silica backup insulation, a firebrick inner shell, and an outer shell of steel. All of the lamps are arranged equidistantly above and below a belt with subsequent lamps being spaced horizontally closer than previous lamps in an attempt to obtain a controllable, sharp temperature rise or spike for optimum reflow conditions at a set belt speed.